The invention is of particular application to the analysis of low volume liquid samples, for example of volumes of 5 microliters or less, such as would be used in the quantitative analysis of aqueous samples of DNA created in the laboratory. Since is such a sample is a valuable resource, only a very small amount is generally available for assessment.
A common method of analysis of such a sample is by spectrophotometry, which involves passing a beam of electromagnetic radiation (for example ultraviolet light) through the sample and measuring the amount of light absorbed at different wavelengths (down to 200 nm).
WO01/14855 shows a liquid photometer in which surface tension is utilised to contain a sample in contact with a lower support surface which is situated opposite an upper support mounted on a swing arm. Such a device is mechanically relatively complex, and the relative moveability of the surfaces results in the requirement that the device needs periodic calibration in view of the possible variations in the length of the path light through the sample over the course of successive uses of the apparatus.
U.S. Pat. No. 4,540,280 shows a fibre optic thin layer cell for use in spectrophotometric analysis of a liquid. In this case, the light passes across a gap defined by the opposed ends of two bundles of optical fibres which are, in effect, cemented to the cell, and are therefore at a fixed position relative to each other. However, the liquid sample has to be fed into the cell through channels, and the cell interior is not readily accessible for cleaning after use.
JP63148144 shows a photometric device whereby the chemical analyser, comprising an optical fibre having a recessed core at one end to provide a volume into which a liquid sample may be placed, to be retained by the portion of the fibre cladding that stands proud of the core. In this case, light is conveyed to and from the sample through the surrounding cladding. Although the path light supplied through the sample is fixed, the volume into which the sample is to be placed can only be accessed through the relatively small tip of the fibre.